Polymeric membranes have steadily evolved over the last 25 years from their infancy in the reverse osmosis and hemodialysis beginnings to recent uses in the separations of gases. Many membranes have proved themselves to be economically viable. Membrane separations initially used dense homogeneous films which exhibited good selectivity but in most instances too low flux to be of economic value. An important hurdle which was overcome in the advancement of membranes was the creation of high flux membranes with the discovery of the asymmetric membrane architecture by Loeb (e.g., U.S. Pat. No. 3,133,132).
Asymmetric membranes provide the selective properties of their dense film counterparts and at the same time provide orders of magnitude improvement in flux. The inherent advantage of asymmetric architecture is that its integral dense "skin" layer is much thinner than a typical solvent evaporated dense membrane. The asymmetric dense skin is supported by a much thicker, non-selective porous layer. The integral support layer is formed along with the dense skin from the same polymer solution in the same process step. The porous layer should be capable of safely carrying the load of the overlying dense skin during pressurized applications.
Many polymers, including the polyacetylenes, have been prepared in asymmetric form using variations of the Loeb procedure. However, as higher and higher productivity is required from an economic perspective, the target thickness of the dense skin layer must be reduced. The problems of producing asymmetric membranes with very thin dense skins are appreciable and become worse as target thicknesses are reduced.
One successful approach around this problem is the subject of the Henis patent (U.S. Pat. No. 4,230,463). where an imperfect asymmetric gas selective membrane is produced with a very thin skin layer which is then repaired by coating with a high permeability, low selectivity polymer. The selectivity and permeability properties of this type membrane are determined primarily by the repaired asymmetric membrane, but in practice is a contribution of properties of both materials. While this type membrane corrects for defects in the gas separating layer, it is a complex multi-step process which is restricted to higher selectivity, lower permeability membrane materials.
U.S. Pat. No. 4,673,418 teaches that improved polyetherimide polymers can be prepared from a solution of the polymer in a solvent having a lower boiling point than any other component in the mixture and an organic liquid swelling agent in which the polymer is not soluble and which does not react with either the polymer or the solvent. The solution is spread to form a film which develops a surface skin by evaporation and the film is then contacted with a precipitation agent to form an integral, asymmetric polyetherimide membrane. The solvents are generally described as halogenated hydrocarbons and the swelling agent as an alkyl substituted benzene, and aliphatic carboxylic acid or chlorinated hydrocarbon. While improved gas permeability is shown for the membranes disclosed in this reference, improved selectivity is not demonstrated.
Tanny, J. App. Poly. Sci., Vol. 18, pp 2149-2163 (1974) and Darcovich and Kutowy, J. App. Poly. Sci., Vol. 35, pp 1769-1778 (1988), discuss the relationship of surface tension of the solvent or solvent mixtures to the resultant asymmetric membrane. Additionally, T. D. Nguyen, et al., Chem. Eng. Comm., Vol. 54, pp 17-36 (1987) discusses the effect of nonsolvent swelling agents in the casting solutions on the average pore size and pore size distribution at the surface of polyimide membranes.
Composite membranes constitute the other primary type of membranes which currently show economic promise. Two basic types of composite membranes include, laminated composite membranes and coated composite membranes. In some cases, composites offer the opportunity to prepare membranes which cannot, because of material cost, availability or other reasons, be otherwise prepared. Both types of composite membranes suffer the same drawbacks as asymmetric membranes do, i.e., thickness of the permselective layer. Ultra-thin skin layers of laminates are very difficult to handle at production scale and are subject to damage at any of the several steps of their manufacture.
Membranes formed from substituted polyacetylenes, have been produced in most of the configurations discussed above: dense films, asymmetric membranes, laminated composites and coated composites.
The substituted polyacetylenes have received much recent interest because of their unique ability to readily form into membranes with reasonable gas selectivity and exceptional permeability (far exceeding that of the next highest competitor, silicone rubber). This unusual property has been proposed to be the result of its high excess free volume associated with the intermolecular spacing of the polymer molecules in the membrane.
Dense films made from substituted polyacetylenes are the subject in part or whole of several recent patents, such as Langsam--U.S. Pat. No. 4,657,564; Langsam, et al.--U.S. Pat. No. 4,759,776; and Higashimura, et al.--EPO No. 0136901. The polyacetylene membranes disclosed in these patents show relatively high permeability and reasonable selectivity, however the flux of any dense, homogeneous membrane is generally low because of overall thickness when compared to that which can be obtained with some of other morphologies from which the membranes can be fabricated.
The existing art of preparing composite membranes with substituted polyacetylenes shows films prepared in both flat sheet and hollow fiber form. In part, references to coated composite membranes for liquid separations (Higashimura et. al. EPO 0136901), coated composite membranes for gas separation (Minoru et. al. U.S. Pat. No. 4,689,267; Higashimura et. al. EPO No. 0136901; and laminated composite membranes (Higashimura Canada 1206681) have been found in the patent literature. As discussed above, all these composite structures suffer from the negative aspects of reliably making very thin gas selective layers.
Several methods for preparing asymmetric polyacetylene permselective membranes have been taught in the art. Details of a phase inversion membrane formation process as it relates to substituted polyacetylenes is provided in Japanese Patent No. 60-132605. All the expected variables such as polymer concentration, solvent/additive make-up, coagulant bath composition and temperature can effect the properties of the resulting membranes. While the primary benefit of the disclosed process is the very high flux resulting from drastically reduced thickness of the asymmetric skin layer, selectivity expectations are, at best, that of the dense solvent cast membrane of the same polymer. Other patents relating to preparation of asymmetric substituted polyacetylene include Higashimura-EP 0136901; and Kenichi-Japan 222111.
Japanese Patent No. 222111 shows improvement in permselectivity, at the expense of flux, with heat treatment for substituted polyacetylene membranes. The patent teaches the benefit for both dense homogeneous as well as asymmetric membranes. This information suggest improvements in selectivity can be realized with appropriate arrangement of the polymer during the membrane formation process. If this restructuring can be localized to the membrane outer surface rather than the whole dense layer, the impact on flux may also be minimized.